This invention relates generally to lawn mowing equipment and more particularly concerns an improved rotary lawn mower of the type in which a cutting blade is mounted for rotation about a vertical axis within a housing which, together with the ground surface, substantially encloses the blade.
Rotary lawn mowers function by impact cutting of blades of grass, that is, by driving the cutting edge to impact the blades of grass at such a substantial velocity that the blades are cut cleanly by the cutting edge before individual grass blades may bend out of the cutting plane. This distinguishes the mowing concept from that of a reel type mower, where blades of grass are cut by means of a pair of cutting edges which operate against one another.
It has been known for some time that impact cutting of grass of various varieties and in varying moisture and toughness conditions may be cleanly made at cutting edge or tip speeds as low as 5,000 per minute (FPM). Despite this knowledge, many prior art mowers have been constructed to operate with their cutting edges moving at a speed of as much as 20,000 FPM. The primary reason for the high blade velocities is that it is desirable to have a lawn mower which will impart sufficient momentum to the cut particles so that they may be collected in a porous bag or other receptacle connected to a discharge opening in the mower housing.
Unfortunately, driving the cutting blade at such high speeds results in substantial noise generation by the engine used to drive the blade and from interaction of the driven blade and mower housing. Driving the blade of a rotary mower at high tip speeds in the range of 20,000 FPM also means that when the blade contacts foreign objects such as stones, wires, or other harder-than-grass debris on the lawn, it is possible to impart sufficient momentum to the debris to cause them to become hazardous projectiles. Since the kinetic energy available for transfer to the foreign objects increases with the square of velocity, it is highly desirable to operate at a lower velocity.
Despite the shortcomings of rotary mowers having blades which are driven at relatively high speeds, mowers available to the consuming public have typically operated at such high blade speeds because of the significant difficulty of constructing a low speed mower which would cleanly cut and bag its cut grass particles, without leaving significant amounts of cut particles on the mown surface or so bruising the grass tips that the lawn takes on a brownish cast within a short time after mowing has been completed. With increased emphasis on safety considerations and sources of noise pollution, there has been corresponding concentration on the problem of constructing a quieter mower with a blade operating at a lower speed without sacrificing other performance features felt to be convenient and desirable by the user, but the problem has not been satisfactorily solved.
The present invention provides a solution to these problems. It is a mower in which a substantially lower blade tip speed is used, yet clean cutting bagging is achieved, even in embodiments using rear discharge openings, which as is well known to those of skill in the art, is considerably more difficult than bagging using a side discharge opening. This is true because cutting of grass occurs in the front two qradrants of blade rotation. Therefore grass must be transported a significantly greater distance for rear than for side discharge. The improved rotary mower of the present invention provides momentum to cut grass particles by means of upward, inward mechanical throwing of cut particles by its trailing sail portions in combination with an axial flow pump effect providing a controlled upward flow of air from the cutting region. Use of an upwardly, inwardly raked sail driven at a relatively low tip speed in contrast to previous machines results in a combined mechanical throwing and fluid pumping action which transports cut grass particles into the expansion channel of the housing.
Although the mechanism of transport has not been analytically defined, it appears to be an entirely different concept of cut particle transport than the very common transport method in which grass particles are cut and thrown essentially tangentially outward to the side wall of the housing where the air turbulence generated by the blade and sail portions thereof transport grass angularly around the side wall to a discharge opening therein.
Another problem at the lower speeds is now more fully appreciated as a result of the present invention. One of the limitations of effective cutting has been discovered to be the problem of keeping the cutting edges "clean" or free of large numbers of cut grass particles which dull the effective cutting edge and result in bruising of grass contacted by the "loaded" cutting edge. As a result of this discovery, certain modifications have been made in specific embodiments of the invention that allow the cutting edges to clean themselves more effectively and thus promote impact cutting at the lower speeds.